This invention provides a blowout-preventer-stack one-trip test tool and method for the oil-and-gas drilling industry.
Drilling for petroleum, especially under water and in deep water, is a very expensive operation, with costs accruing every day whether actual drilling is occurring or not. The cost of suspending drilling operations for required safety testing is immense.
The blowout preventer (BOP) or more precisely the blowout preventer stack of several different types of BOPs is a standard and required piece of safety equipment for oil- and-gas drilling. It is located at the wellhead, which, for deep-water drilling, is at the bottom of the sea. It protects against blowouts caused by kicks or bumps of sub-surface pressure rising from the well.
The drill string is composed primarily of sections of drill pipe surrounded by a casing. The drill pipe moves into and out of the well as drilling progresses. The casing stays in place after it is initially set. Both the drill pipe and the casing are subject to separately varying levels of sub-surface pressure. Drilling fluid or drilling mud is injected into the drill pipe and separately into the casing at closely monitored pressures to counteract the sub-surface pressure. Blowout preventers serve the purpose of sealing off either the casing or the casing and the drill string of the entire well to prevent sub-surface pressure from overwhelming the counteracting pressure of the drilling mud.
Of the various types of blowout preventers in a stack, annulars and fixed and variable rams are designed to seal the casing around the drill pipe while leaving an area to accommodate and not damage the drill pipe. The casing is more susceptible to loss of control of pressure kicks than the drill pipe is, and damage to the drill pipe can cause delays or even complete loss of a well. Blind and shear rams, however, are designed to completely seal off the entire casing, and will damage or shear any drill pipe inside the casing.
Blowout preventer stacks are a regulated and required element of drilling. The regulations require that blowout preventer stacks must be tested frequently and thoroughly. Testing requires that drilling operations be suspended, that the drill string be pulled out of the hole, that a test plug be set at the wellhead, that testing of the rams and annulars be performed, that the test plug be removed, and that the drill string be run back into the hole in order to resume drilling.
Drill pipe is made in typically 30-foot sections, and a drill string has to be assembled at the drilling rig from those sections of drill pipe as the drilling progresses. When the drill string is pulled out of the hole, the sections of the drill pipe have to be disassembled and stacked, and then reassembled on the next trip into the hole. Deep-water drilling requires vast lengths of drill pipe just to reach the wellhead, and then more vast lengths of drill pipe to drill into the seabed. Pulling the drill string out of the hole, running the test plug into and out of the hole, and putting the drill string back into the hole, in deep water, is an operation that can take several days and several cycles of disassembly and reassembly of thousands of sections of drill pipe.
Thorough testing of a blowout preventer stack presently requires more than one trip into the hole, which further delays resumption of drilling operations, because testing of rams fixed for different diameters of pipe require the insertion and removal of those different diameters of pipe, and testing of the blind and shear rams must be performed with no pipe present in the blowout preventer at the wellhead.
For some phases of BOP testing, the wellhead immediately below the BOP stack must be tightly sealed off from the well below, by the test plug, in order to prevent leakage of any pressure coming form or going into the sub-surface well and making it impossible to determine if the blowout preventers are properly holding pressure between each BOP and the test plug at the wellhead. Presently this sealing and unsealing of the test plug at the wellhead requires more than one trip into the hole and carries a risk of not being able to unseal the wellhead and resume drilling operations.
The frequent and thorough testing of blowout preventer stacks is an important safety precaution that is required to be done, but at present, especially for deep-water drilling, the testing of blowout preventer stacks requires long, costly suspensions of drilling operations.